Three reasons for this, I think. Firstly "I got burnt" has various idiomatic usages, like "I got burntby the Nigerian scam". Secondly, it is bad to make the exhaust pipe seem like a volitionalagent (i.e. something that deliberately setsout to burn you). I can imagine a comediantelling a story that relies on the fact that thebike really is out to get him, but it is not normal. Finally, "I got burnt" slightly encouragesthe reading in which after we are done there is nothing left but ashes.

That happened to me once. I was so stupid, at a stop light, absent-mindedly reaching for a piece of string that was hanging on the front, from the fabric bike cover I had taken off earlier. Inside of my left forearm, a nice oval burn. Being such an expert, I would either say: I burned myself on the exhaust pipe; or, I got burned by the exhaust pipe. I'd say the second option doesn't really own up to the goof mistake, though. It's very passive, like you were just standing there and along comes this pipe and burns you. Be a man and take the blame. Use the first option and insert the expletives Ingis recommends in front of "exhaust pipe", since that is what they modify.